It's not so hard to turn humans into living vegetables. Some sorts of head trauma seem to do it. How hard can it be to make that reliable (or at least reasonably reliable) for cows?

Probably not that hard. Doing it without ruining the meat or at least reducing yields sounds harder to me, though -- muscles atrophy if they don't get used, and they don't get used if nothing's giving them commands. I'd also expect force-feeding a braindead animal to be more expensive and probably more conducive to health problems than letting it feed itself.